1. Field of the Invention
The present invention relates to a clip-fastened raceway for a variable compression ratio engine.
2. Description of the Related Art
International patents WO 98/51911, WO 00/31377, WO 03/008783, and WO 07/085,739, belonging to the Applicant, disclose different mechanical devices for a variable volumetric ratio engine and/or a variable cylinder case.
It is observed that international patent WO 98/51911 describes a device which serves to improve the overall efficiency of internal combustion piston engines used at variable load and speed by adapting their effective displacement and/or their volumetric ratio during operation.
This type of engine is also known to one having ordinary skill in the art under the name “variable compression ratio engine”. This term will be used hereinafter.
It is observed from international patent WO 00/31377, that the mechanical transmission device for a variable compression ratio engine comprises a piston which is affixed, in its lower portion, to a transmission member cooperating, on the one hand, with a rolling guide device, and on the other hand, with a toothed wheel affixed to a connecting rod, said wheel allowing movement to be transmitted between the piston and said connecting rod, the latter being articulated around a crank comprised by a crankshaft.
According to the same patent, it can be observed that the toothed wheel further cooperates with a control member whose adjustable vertical position with respect to the engine block, which the variable compression ratio engine comprises, allows for controlling the compression rate of said engine.
It is noted that, according to international patent WO 00/31377, the toothed wheel referred to as a “gear” in said patent comprises segments having a curved profile which are housed in a curved groove housed at mid-width of each toothed sector which said wheel comprises.
The term “runway strip” has thus been retained to designate said segments from international patent WO 03/008783, replaced by the term “raceway” in international patent WO 07/085,739 relating to improvements or alternatives of the same variable compression ratio engine.
It is noted that the raceways comprised by the toothed wheel cooperate with other raceways which comprise, respectively, the transmission member and the control member, the contact point between the raceways and those of said members being positioned in the vicinity of the pitch circle of the gear assembly constituted of said wheel and said members so that said raceways define the center-to-center spacing of said gear, the operating clearance of which they control in the area of the teeth.
International patent WO 07/085,739 also recalls that the minimum operating clearance between the teeth of the racks having large dimensions and those of the toothed wheel is determined by the location of the raceways comprised by said racks and wheel.
According to the same patent, during the operating of the variable compression ratio engine, the raceways of the racks and toothed wheel can be maintained in contact by means of a pressing device constituted of pressure means affixed to the cylinder housing of said engine, said means can in particular comprise a spring and/or a piston housed in a cylinder and subjected to a hydraulic pressure.
It is noted that, according to patent WO 03/008783, the raceways of the transmission and control members are made by machining the bodies which constitute said members. This particular configuration is made necessary by the need to rigidify said members, their rigidity being significantly reinforced when the raceways are integrated to the constitutive material of said members.
However, making said raceways and lateral sides which reinforce the rigidity of said members makes it impossible to use certain usual manufacturing methods for making gear teeth like a hob or a grinding wheel, these circular tools with a large diameter requiring the necessary clearance to operate.
Alternatively, it is for example possible to use precision forging stock of the large teeth of said members, then to high-speed machining or precision electro-chemical machining to finish the teeth and the raceways of said members.
It is noted that the precision electro-chemical machining makes it possible to finish said teeth of said transmission and control members after having cemented and soaked them with a precision on the order of several microns.
It is noted that these different patents do not, however, describe any mode for fastening raceways to the toothed wheel with the exception of international patent WO 00/31377 whose FIG. 6 shows raceways having dovetailed sections engaged in a groove having a complementary shape housed in the toothed wheel.
This solution was never experimented on since its implementation was deemed too complex and potentially would lead to various operational problems.
International patent WO 03/008783 indicates that the electron beam welding can be used as a mode for fastening said raceways on said wheel, this method being used to carry out various physical demonstrators of the variable compression ratio engine being the object of different patents cited in reference.
The main advantages of electron beam welding are that it is moderately complex to carry out, the cost is acceptable, and a low welding energy which limits the volume of thermal matter that is affected.
This low welding energy also limits the permanent set of the toothed wheel caused by welding, said permanent set affecting the geometric integrity and the operational performance of said wheel.
In spite of its advantages, the electron beam welding of the raceways on the toothed wheel presents the disadvantage of having insufficient resistance to fatigue, particularly when the variable compression ratio engine is used with heavy loads.
Various tests of said engine carried out by the Applicants of the present patent application have led to the partial or total unsoldering of the raceways fastened to the toothed wheel according to this method.
The breaks of said raceways subsequent to the unsoldering, accompanied, or not, with the total or partial detachment of said raceways from the toothed wheel, have led to the deterioration or even the destruction of said engine depending on whether said detached raceways or raceway portion are passed, all or in part, into the gear system of said engine.
In addition, the electron beam welding requires the raceway to be adequately flattened against the toothed wheel at the time of welding, without which said welding would be too strongly stressed during the operation of the variable compression ratio engine. In addition, electron beam welding requires the portion of the toothed wheel exposed to welding to be masked during the cementation phase, or an overthickness of the material to be removed by machining after cementation and before welding, in the welding zone to be provided.
Indeed, the electron beam welding is incompatible with a high carbon ratio of the steel constituting the toothed wheel or that constituting the raceway to be welded. The necessary masking is not well-adapted to the industrial production of said wheel since it is too complex and expensive.
As an alternative to electron beam welding, the possibility of securing the raceways onto said wheel by laser welding, friction welding, induction welding, brazing, or any other type of welding or fixed connection known to one having ordinary skill in the art, including any type of elastic fastener, was evaluated.
These more or less complex implementation methods have various drawbacks, and particularly those connected to the deformation of the pieces welded together or to an insufficient resistance to fatigue of the welding.
Other methods have also been envisioned to secure the raceways on the toothed wheel of the variable compression engine and particularly that which consists in making the toothed wheel by forged half-wheels, one of said half wheels directly integrating the raceways of said wheel at the time of forging, and the two half wheels being subsequently assembled to one another as claimed in international patent WO 03/008783, the description of which clearly indicates that the assembly of said two half wheels can be carried out, for example, by welding of any type, and particularly by electron beam or by laser, or by any brazing method, and particularly electromagnetic induction brazing.
The latter method, consisting in one of the half wheels carrying raceways integrated to its material, provides excellent hold for said raceways on the toothed wheel.
In addition, said method excludes any problem of flattening said raceways onto said wheel necessary at the time of welding said raceways onto said wheel since there is no longer any welding, but also during the grinding of said raceways once mounted on said wheel, or when the variable compression ratio engine is operating.
To the contrary, the principle of the half wheel carrying the raceways integrated to its material present the drawback of limiting the fabrication methods usable to finish the teeth of the toothed wheel because the presence of said raceways on said wheel very early on the machining range excludes the use of a wheel or milling or grinding cutter to finish said teeth.
Alternatively, a cutting by means of a knife and a shaving wheel finishing can be used, these two methods being in theory cheaper and less precise than the more conventional gear machining tools.
In addition, the material used for the raceways cannot be different from that used to make the toothed wheel since said raceways are made from the same piece of material than said wheel, which does not allow for optimizing it as a function of the specific needs adapted to said raceways.
It is the same for surface treatment and thermal treatment of said raceways which can hardly be different from those of the toothed wheel, said raceways being definitely affixed to said wheel.
Another method can consist of decreasing the outer diameter of the raceways of the toothed wheel so that said diameter is substantially less than the diameter at the bases of the teeth comprised by said wheel.
This method simultaneously allows the integration of said raceways in the material of the toothed wheel and the use of a cutting or grinding wheel to make the teeth of said wheel.
The drawback of this method is that it moves the pitch circle of said gear system away from the contact surface of the different raceways which said system comprises.
This moving away introduces an important gliding component in the rolling area operated at the contact point between the different raceways of said system, said component leading to an increase of the energy dissipated by friction by said system, and said increase diminishing the mechanical efficiency of the variable compression ratio engine.